Travelling wave tube



June 5, 1962 R. I. HARRISON TRAVELLING WAVE TUBE 2 Sheets-Sheet 1 Filed Feb. 26, 1957 6 y w/w a e a I 6 a a I .QIILMM a Q a a i M a W 1 M 1 q Mpg/M 0 4@ 8 0 e W M A 0 u r r r ill i I I I l N w mm A mH V ml. 0 m H m R BY W ATTORNEY I June 5, 1962 R. 1. HARRISON 3,038,100

TRAVELLING WAVE TUBE Filed Feb. 26, 1957 2 Sheets-Sheet 2 Fig.4a/

/02 /04 /02 /04 loz" m4 m5 INVENTOR RICHARD J. HARRISON A'ITORN United States Patent ()fiice 3,%8,i@ Fatented June 5, 1962 3,038,1itltl TRAVELLING WAVE TUBE Richard I. Harrison, Mineola, N.Y., assignor, by mesne assignments, to Sylvania Electric Products Inc, Wilmington, Deh, a corporation of Delaware Filed Feb. 26, 1957, Ser. No. 642,394 3 Claims. (til. 315-345) My invention relates to travelling wave tubes.

The conventional travelling wave tube depends for its characteristics upon the interaction between the field of an electromagnetic wave propagated along a slow wave structure, such as a helix, and a beam of electrons travelling with the wave, as for example, by travelling axially within the helix. The slow wave structure is periodic in the direction of beam travel. The velocity of field propagation along the structure (the synchronous velocity) is adjusted to be slightly less than the velocity of the beam. Due to the resultant periodic electromagnetic interaction between the beam and the helix, the power is transferred from the beam to the hel'm. The travelling wave tube can be used as an amplifier or as an oscillator.

It is known to the art that an electron beam can function as a special type of transmission line capable of propagating slow electromagnetic fields, (A detailed analysis of this type of beam function can be found, for example, in an article written by R. Kompfner entitled Travelling Wave Tubes and published in 1952 as Repts. Progr. Phys. 15,275.)

I have discovered that when a slow electromagnetic field propagating along a first electron beam is periodically shielded from a second electron beam travelling in a direction parallel tothe first beam, or stated dif ferently, when the two beams are periodically coupled to each other, these two beams interact in the same man:

nor as if the first beam were replaced by a conventional slow wave structure. Thus, I have invented a new type of travelling wave tube which, instead of requiring the combination of a slow Wave structure and a single electron beam, only requires two periodically coupled electron beams.

My rtube can be produced more rapidly and less expensively than conventional travelling wave tubes. Moreover, my tube can be used either as a forward wave tube or as a backward wave tube and, further, can be used either as an amplifier or as an oscillator.

Accordingly, it is an object of the present invention to substitute, for the conventional travelling wave tube combination of slow wave structure and single electron beam, two periodically coupled electron beams.

Another object is to provide a new and improved travelling wave tube characterized by the absence of a slow wave structure.

Still another object is to provide a new and improved travelling wave tube wherein gain is obtained by the transfer of energy between two periodically coupled electron beams.

Yet another object is to provide a new and improved travelling wave tube employing, two periodically coupled electron beams, the direct current power associated with, one of said beams propagating in the same or opposite direction with respect to the propagation direction of the direct current power associated'with the other beam.

These and other objects of my invention will either be explained or will become apparent hereinafter.

As is well known to the travelling wave tube art, when an electromagnetic field of given frequency and given. mode propagates in a given direction along a slow wave structure which is periodic in the direction of beam travel, the field can be decomposed into an infinite number of spacial harmonic wave components which have the same given frequency. Each of the powers associated with each component flows in said given direction and can be added together to produce the total power. However, the direction of propagation for these components varies such that the phase velocity vector of each component points either in the given direction or an opposite direction; in general, at any given frequency, each component has a different phase velocity. The fraction of the total power carried by each component is a constant determined by the characteristics of the particular slow wave structure used. In particular, the component of highest power level which has its phase velocity vector pointing in the direction of power flow is denoted as the h g or fundamental component, and the component of highest power which has its velocity vector pointing in a direction opposite to that of the power flow is denoted as h or backward wave com ponent.

Travelling wave tubes can be segregated into two main types; the forward wave type in which the electrons travel in the direction of wave energy propagation; and the backward wave type in which the electrons travel in a direction opposite to the direction of wave energy propagation.

The gain of a forward wave tube remains essentially constant over a wide frequency range, and hence it is extremely useful as a broad band amplifier. However, because of this gain-frequency characteristic, the forward wave tube is not well adapted for use as an oscillator or as a narrow band amplifier.

In contradistinction, the gain of backward wave tubes is only constant over a narrow frequency range. Further,

while the backward wave tube will amplify low level signals over this narrow range, since it inherently is a regenerative type device, it presently finds its principal application as an oscillator.

In the conventional forward wave tube, the beam velocity is made approximately equal to the phase velocity of the fundamental component. The interaction of the beam and the fundamental component (since the beam and this component travel in the same direction) results in a growing Wave (i.e. increased wave amplitude) in the direction of the beam thereby producing the desired gain.

On the other hand, in the backward wave tube the beam velocity is made approximately equal to the phase velocity of the backward wave component. The resultant beam-component interaction is regenerative in nature, since the beam carries energy in a direction opposite to the direction of power flow of the backward wave component while the phase velocity of the beam and the backward wave component point in the same direction and are approximately equal (or synchronous). Therefore, positive or regenerative feedback takes place over each differential length of the beam.

In accordance with the principles of my invention, first and second electron beams are caused to travel in essentially parallel paths. An electromagnetic wave of given frequency propagates along the first beam. Means associated with said beams periodically shields the second beam from the field produced by this wave, or, stated differently, periodically couples this field to the second beam. The interaction between the second beam and the field is equivalent to the interaction between a beam and the field propagating along a helix as displayed in a conventional travelling wave tube.

Consequently, the periodically coupled field can be decomposed into an infinite number of spacial harmonic wave components, all components carrying power in the same direction (the direction in which the field propagates along the first beam), the phase velocity vectors of all components in a first set pointing in this direction, the phase velocity vectors of all components in a second set pointing in an opposite direction. Each component is contained in one of these first and second sets. The field and all of its components have the same frequency. The component of the first set having the lowest phase velocity is known as the fundamental component. The phase velocity of each other component has some fixed ratio to the velocity of the fundamental component.

For any given frequency, the Velocity of the first beam will be approximately equal to the phase velocity of the fundamental component, and thus the velocity of the first beam establishes the phase velocities of all the wave components.

Hence, in my invention, when both first and second beams travel in the same direction, and the velocity of the second beam is adjusted to be approximately equal to the phase velocity of a selected component in the first set, as, for example, the fundamental component, a forward wave tube interaction ensues. Conversely, if the first and second beams travel in opposite directions, and the velocity of the second beam is adjusted to be equal to the phase velocity of a selected component in the second set, as, for example, the backward wave component, a backward wave tube interaction ensues.

Illustrative embodiments of my invention will now be described in detail with reference to the accompanying drawings wherein FIGS. 1 and la illustrate one embodiment of my invention;

FIGS. 2 and 2a illustrate a second embodiment of my invention;

FIGS. 3 and 3a illustrate a third embodiment of my invention; and

FIGS. 4 and 4a illustrate a fourth embodiment of my invention.

Referring now to FIG. 1, there is provided a travelling wave tube structure having an evacuated envelope (not shown) andan electrically conductive cylinder 35 disposed within the envelope. First and second separate coupling means or transition elements 40 and 42 are connected to cylinder 35. A first electron beam 18 is generated at electron gun 14 and travels within cylinder 35 toward collector mounted within element 42 at a velocity V A second electron beam 26 is generated at electron gun 16 and travels within cylinder 35 in a direction opposite to beam 18 toward collector 12 within cylinder 35 in a direction opposite to beam 18 toward collector 12 mounted within element 40 with velocity VB A plurality of parallel, electrically conductive members 44 equidistantly spaced apart from each other are connected at both ends to the cylinder 35 and are interposed between the two beams, each member extending in a direction perpendicular to the directions of beam travel, as shown in more detail in FIG. 1a.

The voltages of batteries 26 and 28 determine the beam velocities V and V respectively. Similarly, the voltages of batteries 30 and 32 respectively determine the beam currents of beams 18 and 20. Note that cylinder 35 is coupled to the junction of batteries 26 and 28 and hence is maintained at a positive potential with respect to the two electron guns.

As is conventional, these beams are columnated by an axial magnetic field. For the purposes of clarity, the external magnets for producing this field, as well as internal gun mounting structures and the like, which are common both to my tube and conventional tubes, have been omitted from all the figures of this application.

An electromagnetic wave of given frequency and mode is supplied to coupling element 40 and thereafter propagates along beam 18.

At each member 44 the field produced by the propagating wave is periodically shielded from beam 20. At positions intermediate the member, the field is periodically coupled to beam 20,

The resultant action With respect to beam 20 is the same as if beam 18, cylinder 35 and members 44 were replaced by a conventional periodic slow wave structure.

As indicated previously, the periodically coupled field can be decomposed into an infinite number of spacial harmonic wave components having the same frequency as the incoming wave. The phase velocity vectors of a first set of these components point in the direction of beam 18; the phase velocity vectors of a second set point in the direction of beam 24 Each component is contained in one or the other of these two sets.

The velocity V of beam 18 establishes the phase velocity of the fundamental component of the first set and, as previously discussed, thus determines the phase velocities of all other components. The velocity V of beam 26 is then adjusted to be approximately equal to the phase velocity of a selected component in the second set, as for example the backward wave component, and the tube then functions as a backward wave amplifier, the output signal appearing at element 42.

It will be apparent that by applying the incoming wave to element 42 and interchanging the velocities of beams 18 and 20, the tube will function in the same manner but the output signal will appear at element 40.

In the event that forward wave operation is desired, beams 18 and 20 are caused to travel in the same direction rather than opposed directions as shown here. The velocity V of beam 18 then establishes the phase velocities of the various components as before. In this case, however, the velocity V of beam 20 must be adjusted to be approximately equal to the phase velocity of a selected component in the first set, as, for example, the fundamental component, and the tube will function as a forward wave amplifier. (Note that in this example both beams have the same velocity V When the tube shown in FIG. 1 is to be used as a backward wave oscillator, no incoming wave is applied to either coupling element 40 or 42. However, as is known to the art, all wave guides and beams generate a noise spectra. Due to the regenerative action of the tube, the frequency component of the noise spectra equal to the selected frequency is selectively amplified. The gain is in general a function of the combined beam currents; when both beam currents are increased to a point such that the gain becomes infinite, the tube of FIG. 1 oscillates.

It will be apparent to those skilled in the art that many other arrangements for periodically coupling or shielding the two beams from each other can readily be used. For example, the round members can be replaced by an electrically conductive plate having equidistantly spaced circular or elliptical holes through which the beams can be coupled together, or a series of equidistantly spacial dielectric obstacles can be used to periodically shield the two beams from each other.

FIG. 2 shows an arrangement in which the beam 18 is a hollow beam and beam 20 is a solid beam which travels along the axis of beam 18. "In this example, the periodic coupling is obtained by equidistantly separated, electrically conductive rings 50. The operation of the device of FIG. 2 is substantially identical with the device of FIG. 1.

In the device of FIG. 3, the two electron beams are intermingled in the main portion of their path, these beams being separated at points adjacent the guns and collectors by the action of magnetic field beam separation devices 36 and 38. The magnetic flux lines established by these devices are directed along a line extending perpendicularly into the paper, and because of the opposed directions of travel of the beams in the region where these fields are effective, the separation occurs in the manner indicated.

A plurality of equidistantly spaced magnetic lenses 52 are used in place of the members 44 of FIG. 1. In regions where these lenses have no appreciable effect upon the intermingled beams, i.e. at regions intermediate the lenses,

aoasnoo the intermingled beam cross section is relatively large (as shown in more detail in FIG. 3a), and there is essentially no coupling between the two beams. However, in regions where the lenses are effective, the intermingled beam cross section is relatively small, and a high degree of coupling is obtained. Since the coupling is periodic, the device of FIG. 3 functions in essentially the same manner as that of FIG. 1.

FIG. 4 shows a variation of the device shown in FIG. 3, wherein the beams are introduced into the metal cylinder 35 at an acute angle with respect to the axis of the cylinder. A plurality of equidistantly spaced pairs of magnets 100 are placed along the cylinder in the manner shown in FIG. 4a., the two magnets 102 and 104 in each pair having opposite polarities, and any two corresponding magnets 102 and 104 in any two adjacent pairs 100 having opposite polarities. The two beams then travel in the paths shown in FIG. 4 and snake or wiggle past each other, coupling being obtained at the periodically spaced points 106 where the two beams are intermingled, the intermingled beam cross section at these points being small. The tube of FIG. 4 functions in essentially the same manner as that of FIG. 1.

The paths of the two beams as shown in FIGS. 3 and 4 are not precisely parallel. However, since the amplitude of the beam snaking action is relatively small compared to the length of the path between gun and collector, these paths are essentially parallel to each other.

It Will be apparent to those skilled in the art that utilizing the principles of my invention, the two beams can flow in curved rather than straight paths as used for example in magnetrons and in the particular types of backward wave tubes known as Matype carcinotrons.

While I have shown and pointed out my invention as applied above, it vw'll be apparent to those skilled in the art that many modifications can be made within the scope and sphere of my invention as defined in the claims which follow.

What is claimed is:

1. A travelling wave tube comprising a hollow electrically conductive cylinder provided with spaced input and output connections; a first electron beam travelling from said input connection to said output connection along a given path through said cylinder with a first velocity; an electromagnetic field of given frequency propagating on said first beam; a second electron beam travelling from said output connection to said input connection along a path essentially parallel to said given path with a second velocity, and a plurality of magnetic lenses spaced along said conductive cylinder, said magnetic lenses periodically coupling said first electron beam to said second electron beam.

2. A travelling wave tube comprising a hollow electrically conductive cylinder provided with spaced input and output connections and having first and second ends; first and second beam separation means affixed to the first and second ends respectively of said conductive cylinder; a first electric beam travelling with a first velocity from said input connection to said output connection along a given path through said cylinder; an electromagnetic field of given frequency propagating on said first beam; a second elec tron beam travelling with a second velocity from said output connection to said input connection along a path essentially parallel to said given path; said first and second electron beams being separated at said first and second ends by said first and second beam separation means; and a plurality of magnetic lenses spaced along said conductive cylinder, said magnetic lenses producing an axially magnetic field for periodically coupling said first electron beam to said second electron beam.

3. A travelling wave tube comprising a hollow electrically conductive cylinder having first and second ends, means for injecting first and second electron beams at first and second velocities into the first and second ends of said cylinder respectively; each of said beams being injected at an acute angle with the longitudinal axis of said cylinder, an electromagnetic field of given frequency propagating on said first beam, and a plurality of magnets spaced along said conductive cylinder, said magnets producing a transverse magnetic field for periodically coupling said first electron beam to said second electron beam.

References Cited in the file of this patent UNITED STATES PATENTS 2,683,238 Milhnan July 6, 1954 2,684,453 Hansell July 20, 1954 2,730,647 P-ierce Jan. 10, 1956 2,741,718 Wang Apr. 10, 1956 2,757,311 Huber et 'al. July 31, 1956 2,794,146 Warnecke et a1 May 28, 1957 2,830,271 Pierce Apr. 8, 1958 2,911,556 Charles et a1 Nov. 3, 1959 2,926,281 Ashkin Feb. 23, 1960 FOREIGN PATENTS 1,080,230 France May 26, 1954 1,106,301 France July 20, 1955 706,094 Great Britain Mar. 24, 1954 

